Description: The Health Plan Employee Data and Information Set 'HEDIS' is a set of standardized performance measures initially designed to enable purchasers of health care to evaluate and compare a managed care plan’s administrative performance. The National Committee for Quality Assurance, known as the NCQA, developed HEDIS in 1991. Since its introduction, HEDIS has become a widely used tool for measuring a broad array of health plan functions. This study evaluates the reliability and validity of HEDIS measures. It also reviews the usefulness of HEDIS as an indicator of health care quality.